This invention relates generally to actuating screws and more particularly to a holddown lock screw as is commonly used in pressurized wellhead structures for such functions as holding down a casing or tubing hanger mounted in the wellhead or for energizing a pack-off seal or any of a variety of wellhead seal devices.
In a typical wellhead structure, one or more concentrically disposed casing strings are suspended from casing hangers which are supported within the bore of a casing head member. A hanger bowl lowered into the wellhead provides support for a tubing hanger which receives one or more tubing strings extending down into the well within the innermost casing string.
The hangers are normally held down by a plurality of holddown screws which are threaded into the wellhead and against camming surfaces of the hangers in order to urge these components downwardly and prevent their blowout from the well. In other applications, holddown screws are used to provide a preload for energizing pack-offs or other wellhead seal devices. Typically, a plurality of holddown screws are arranged with equiangular spacing in co-planar disposition about the wellhead and the object to be held down or energized. Each of the screws is adapted to be moved axially in a radial direction towards the longitudinal axis of the wellhead bore. With existing designs this is accomplished mechanically whereby the torque applied by a wrench to the screw shaft is converted into thrust by means of screw threads. Since a number of screws are employed, torque must be applied to the screws alternately and incrementally. This procedure is time consuming, difficult and usually requires at least two persons and the use of a long "cheater" bar. If the torque is not applied evenly in an alternate manner to each of the holddown screws and in small enough increments the hanger to be held down or seal to be energized will "cock". Then, regardless of the amount of subsequent load applied the seal cannot be brought back parallel. As a result the sealing surfaces will not contact and leakage will result. If this happens the sealing surfaces may also be damaged so that replacement of the seal is necessary. This problem is maximized with metallic seals.
It is therefore a primary object of the invention to provide a holddown screw assembly which may be energized hydraulically or mechanically and in an application employing a plurality of holddown screws may be energized simultaneously in concert with each of the other holddown screws whereby each of the screws evenly applies a load to the seal to be energized.
It is another object to provide a holddown screw which may be energized by hydraulic means and then mechanically retained to permit removal of the hydraulic means.
It is a further object of the invention to provide a holddown screw assembly of a plurality of holddown screws arranged in co-planar equiangular spacing about an object to be actuated or energized wherein the plurality of screws may be energized simultaneously by hydraulic means and then mechanically retained.